Analog-to-digital converters (ADCs) often include a plurality of input/output paths (channels) in which input signals are processed to produce corresponding output signals. Interleaved and multi-channel ADCs are prone to performance degradation due to cross-coupling between channels. Causes of performance degradation include kick-back and memory effects, both of which can be more severe compared to single-channel ADCs.
FIG. 1 shows a block diagram of a conventional multi-channel pipelined ADC. For illustration purposes, only two channels are shown. However, it will be understood that any plurality of channels may exist. A first channel 105 and a second channel 205 may be identical with respect to the structure and interconnections between respective stages in each channel. Each channel may include any number of pipelined stages. In a multi-channel ADC, each channel may operate independently. For example, channel 105 may sample and convert a first data signal while channel 205 samples and converts a second data signal. In an interleaved ADC, the channels may sample the same data signal in an alternating fashion and the outputs of all channels may be combined to provide a higher effective sampling rate compared to sampling the data signal using a single channel.
In the channel 105, three stages 100/110/120 are connected in succession so that the output of one stage may serve as the input of the next stage. For illustration purposes, only the first two stages and the final (Nth) stage are shown. However, any number of stages can be connected in this fashion. The first stage 100 is shown in detail and is connected to an analog input voltage Vin and includes an ADC 10 (also known as a “flash”) and a multiplying digital-to-analog converter (MDAC) 50. The MDAC 50 includes a digital-to-analog converter (DAC) 20 and an amplifier 30. Vin is input to the ADC 10 to generate a digital input to the DAC 20, which in turn converts the digital output of the ADC 10 back into an analog signal. The analog output of the DAC 20 is then subtracted from Vin to obtain a residue signal, which is then input to the amplifier 30 to generate an analog output voltage VO as input to the next stage, i.e., stage 110. The stages 100/110/120 may include similar components, with the analog output of one stage going into the input of the next stage in order to perform an analog-to-digital conversion of Vin. However, the final stage, i.e., stage 120, may not include a DAC or amplifier since the final output of the ADC is a digital signal that can be generated, for example, directly from the output of the ADC 10.
In a single-channel ADC, a set of capacitors may be charged by an input signal source during a sample phase, then switched to being connected to a reference voltage source during a hold (amplify) phase. When the capacitors are switched back to the input signal source during the next sample phase, residual charge stored on the capacitors can become superimposed onto the value of the input signal source in this sample phase. Part of this “kick-back” can be sampled by the input network at the end of this sampling phase. The kick-back is non-linear in that the amount of charge injected back into the ADC is not a linear function of the value of the input. Thus, it is not possible to correct for the kick-back using only the input value, e.g., by scaling the input value.
Kick-back can also have adverse effects in a multi-channel or interleaved setting, leading to performance degradation in both signal-to-noise ratio (SNR) and spurious free dynamic range (SFDR), as well as degradation in small signal linearity because of integral nonlinearity (INL) jumps. Two types of kick-back can occur due to cross-coupling. The first type is kick-back due to the input signal sampled on one channel's capacitances being transferred onto sampling capacitances of another channel. The second type is kick-back due to the charges on the capacitors of the DAC 20 being transferred to the sampling capacitors of another channel. This may occur, for example, when the capacitors in the DAC 20 are also used as sampling capacitors for the other channel and are not reset before sampling by the other channel.
In addition to the sharing of DAC capacitors, the sharing of components, such as amplifiers and capacitors, in other locations along the channels may also produce coupling. For example, the amplifier 30 may be shared between the first stage 100 in the channel 105 and the first stage 200 of the channel 205. In another example, a feedback capacitor in the amplifier 30 of the first stage 100 in the first channel 105 may be shared with an amplifier of the first stage 200 in the second channel 205. This is possible because usage of the shared components is alternated between channels. Thus, the channels 105/205 may be coupled via the shared components so that, in addition to the kick-back effects described above, a memory effect is produced in which the previous sample in one channel contaminates the current sample of another channel, leading to inter-stage errors that reduce SNR, SFDR and small signal linearity.
In an interleaved ADC, cross-coupling effects may limit the number of interleavable channels to no more than two. With two interleaved channels, the channels perform sampling during alternating clock cyles, so that no overlapping samples occur. However, with three or more interleaved channels, samples may overlap, causing glitching (e.g., kick-back) to occur during sampling. Thus, operation may be required to be restricted so that no channel switching occurs while the input is being sampled on any particular channel.
In a multi-channel ADC, components are not usually shared. However, inter-channel cross-coupling may still occur through other coupling mechanisms. For example, cross-coupling can occur when channels are located on the same substrate or circuit board, are connected to common reference signals such as ground, or located within close proximity to each other. Although cross-coupling effects are not as severe compared to interleaved ADCs (since the outputs of the channels are usually independent), performance can still be noticeably affected.